Re: [PATCH] nilfs2: fix possible int overflows in nilfs_fiemap()

[Ryusuke Konishi]

On Thu, Jan 16, 2025 at 2:08 AM Nikita Zhandarovich wrote:
>
> Since nilfs_bmap_lookup_contig() in nilfs_fiemap() calculates its
> result by being prepared to go through potentially
> maxblocks == INT_MAX blocks, the value in n may experience an
> overflow caused by left shift of blkbits.
>
> While it is extremely unlikely to occur, play it safe and cast right
> hand expression to wider type to mitigate the issue.
>
> Found by Linux Verification Center (linuxtesting.org) with static
> analysis tool SVACE.
>
> Fixes: 622daaff0a89 ("nilfs2: fiemap support")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Nikita Zhandarovich <n.zhandarovich@xxxxxxxxxx>
> ---
>  fs/nilfs2/inode.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/fs/nilfs2/inode.c b/fs/nilfs2/inode.c
> index 23f3a75edd50..81abb58dcbd8 100644
> --- a/fs/nilfs2/inode.c
> +++ b/fs/nilfs2/inode.c
> @@ -1188,7 +1188,7 @@ int nilfs_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
>                         if (size) {
>                                 if (phys && blkphy << blkbits == phys + size) {
>                                         /* The current extent goes on */
> -                                       size += n << blkbits;
> +                                       size += (u64)n << blkbits;
>                                 } else {
>                                         /* Terminate the current extent */
>                                         ret = fiemap_fill_next_extent(
> @@ -1201,14 +1201,14 @@ int nilfs_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
>                                         flags = FIEMAP_EXTENT_MERGED;
>                                         logical = blkoff << blkbits;
>                                         phys = blkphy << blkbits;
> -                                       size = n << blkbits;
> +                                       size = (u64)n << blkbits;
>                                 }
>                         } else {
>                                 /* Start a new extent */
>                                 flags = FIEMAP_EXTENT_MERGED;
>                                 logical = blkoff << blkbits;
>                                 phys = blkphy << blkbits;
> -                               size = n << blkbits;
> +                               size = (u64)n << blkbits;
>                         }
>                         blkoff += n;
>                 }

Thank you Nikita.

I'll take this patch and send it upstream.

Since nilfs2 extents (contiguous blocks) cannot cross segment
boundaries, these overflows do not occur in normal format.  However,
in an environment where the segments are tuned to be larger, these
overflows can theoretically occur, so as you pointed out, I believe
this fix is necessary.

Thanks,
Ryusuke Konishi